Current switching circuits selectively enable or inhibit current flow in response to a signal voltage. Some circuits, e.g. phase detectors and voltage controlled oscillators, require highly accurate current switching circuits for their input. In these applications the accuracy and settling time of the output current is particularly critical.
Present current switching circuit arrangements typically involve the switching or steering of a predetermined current between two branches. Such an arrangement may be too limited in its speed, due to the required charging time for resistors and parasitic capacitances. Also, the output current actually flows through the switching devices. As a result, there is significant current leakage in both the branch carrying current and the branch from which the current was switched. This leakage is not entirely predictable for a given circuit and therefore can effect the accuracy of the output. Moreover, because of voltage drops across the switching devices, dynamic range is sacrificed. Finally, switching the current from one branch to another is likely to result in switching transients which add directly to the output current. These increase the time required for validation of the output signal.